Some places have rich, loamy, fertile soil that can grow almost anything; some places have hard, clay-filled dirt that roots have to struggle to break through; some places have barely any dirt and are mostly exposed rock. What drives this difference? Is it possible to look at a place’s geography and predict its soil type?
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Soil is made of two things: tiny pieces of rock, and biological stuff both living and dead.
Rock gets weathered (broken into smaller pieces) and eroded (the pieces are moved away) by many different processes, like wind and water. Which processes happen, as well as what the rocks are actually made of in the first place, determine the type of pieces they end up as. Clay is really small pieces, and sand is small but not as small as clay, for example. The grains can also have different shapes, like spiky or smooth. Size and shape determine how compact the dirt can be, how much space for other stuff there is between the grains.
The biologic stuff is even more complicated. Plants grow roots, the roots die and decay, bugs live and die, bacteria do all sorts of stuff to all that, burrowing animals move it around, large animals eat the plants and defecate; all of this biological matter goes into making soil.
Different places have different soil because they have different rocks, different climates, and different ecosystems. One can indeed look at all of these things and predict the soil type, but only in very broad categories. Details can change a lot over a very small area. Even small farms can have different soil types in different parts of their field.
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